having trouble getting onto the wings is a new one to me… [edit: im being harsh…but man up and save Mono!]

are you guys trying to aim at them and jump forward? I just run along side and jump sideways always works.
Oh, and its far easier to either run in line with 13 while firing arrows, or just to stop under it when it turns. If you fall off, get back on the horse and hold L1 to find it before it emerges. get to a place where you start to circle and stop, then just shoot arrows into the sections as they emerge from the ground right in front of you.

If you think that jump is hard try and mount Agro in the field while running, its my favourite trick to do, little dissapointed it wasnt part of the trophy.
Simply run away from Agro, hold X and then time your jump and press R1 to mount.
You'll climb up and continue going at full gallop.
I use it whenever ive been picking fruit or lizards and Agro's gone for a walk, but it is epic when used during the colossi battles.